This invention relates generally to an automatic level control system and method. In a particular application, the present invention relates to such a system and method for continuously maintaining the level of dry, powdered material between lower and upper limits in a surge tank prior to mixing the material into a slurry to be pumped into an oil or gas well.
In the oil and gas industry, cement slurry is mixed at a well site prior to the slurry being pumped downhole for cementing a tubing string, such as casing, in the well, for example. Other types of mixtures can be made and used at well sites. Considering cement by way of example, powdered cement is typically added in controlled amounts into the slurry mixture from a surge tank into which bulk powdered cement is added to replenish the supply in the surge tank. It is desirable to keep at least a minimum level of the dry cement in the surge tank to assist the gravity flow of the cement out of the surge tank. It is desirable to keep the level of the dry cement below a maximum level to prevent overflowing the surge tank.
In the past, one type of surge tank has had two sight glasses, or windows, through which a human operator could look to see if the level of the cement in the tank was between the lower and upper limits defined by the locations of the sight glasses. If the operator were to see the material drop below the lower sight glass, an inlet valve would be opened to allow more material to flow into the surge tank. If the operator were to see the material rise above the upper sight glass, the valve would be closed to stop the flow of material into the surge tank. In this manner the level of material in the surge tank would be contained between the defined limits. A disadvantage of this technique, however, is that the material often coats the sight glasses, thereby making it difficult to determine when the material actually drops below the lower sight glass or rises above the upper sight glass.
In another type of control for a surge tank, a single feedback loop is used. For example, the weight of the surge tank is sensed and a responsive signal is generated to adjust the position of a valve controlling the input of the material into the surge tank. The single feedback loop functionally comprises sensing the weight and generating a valve control signal in response, typically to either open or close the valve.
There is the need for an improved system and method with which to control the flow of material into the surge tank, and thereby the level of the supply of material in the surge tank. Such system and method should provide for continuously controlling an inlet valve when the level of the material in the surge tank is between lower and upper limits. Such system and method should also utilize a main control loop generally of the type described above, but in conjunction with a nested control loop that insures the inlet valve is at the desired position indicated by the main control loop. Such a system and method should also be readily adaptable to control materials of different densities.